Referring to FIG. 1, the conventional weaving loom primarily comprises a warp reel 1, a harness 2, a shuttle 3 a grill or reed 4 and a cloth roll 5. Ordinarily, woven fabric is produced by combining a plurality of warp yarns and a weft in a particular relation, such as in an interlacing relation. Normally, the plurality of warp yarns are divided into upper warp yarns 6a and lower warp yarns 6b. The upper warp yarn 6a and the lower warp yarns 6b are respectively held by an upper harness 2a and a lower harness 2b such that a shed is formed between the upper warp yarns 6a and the lower warp yarns 6b. In weaving operation, a weft-carrying insertion member, which is also known as a shuttle 3 (hereinafter "shuttle"), moves back and forth through the shed in a particular motion. Whenever the shuttle 3 completes a cycle, the grill (i.e., reed) 4, which acts as a comb, beats the weft to the right side to make the woven fabric dense and then returns to the left. A weaving loom of this kind is a so-called traditional weaving loom.
By contrast, a shuttleless weaving loom uses a weft-carrying gripper instead of a shuttle, for example, in the rapier weaving loom. Referring now to FIG. 2, a schematic diagram of a shuttleless weaving loom is shown. A weft-carrying gripper 30 and a weft-drawing gripper 31 are carried by a flexible strap 33 which is wrapped around a wheel 36. The wheel 36 ( for example, a belt wheel or gearwheel) is in turn driven by a driving mechanism 35. During weaving, the weft-carrying gripper 30 and weft-drawing gripper 31 move in a certain reciprocating motion, for example, in a more or less sinusoidal motion (also known as simple harmonic motion). To obtain such a motion, the driving mechanisms for the non-traditional weaving loom are mainly of three types: jet loom, gear and crank loom, and variable pitch cylindrical cam loom. Of the above three types of weaving machines, the variable pitch cylindrical cam loom is increasingly popular due to its high insertion rate of the weft end.
Genini U.S. Pat. No. 4,052,906, discloses a mechanism for controlling the motion of the weft-carrying grippers in looms. With reference to FIG. 3, the mechanism employs a cylindrical cam 29 with two pairs of cylindrical wheels 28 moving on each thread of the cam screw 29. In such a mechanism, the cylindrical cam 29 is hard to manufacture and machine. Furthermore, the cylindrical wheels 28 suffer serious wear at high insertion rates. See FIG. 4, wherein Ohmura, U.S. Pat. No. 4,535,642 is diagramatically shown. The traversing mechanism of Ohmura's patent is primarily composed of a cam shaft 41 and a cam shoe 40. The cam shoe 40 is mounted freely slidable on the helical cam groove of the cam shaft 41 for reciprocating a guide member 42. In such a traversing mechanism, the cam shoe 40 and the cam shaft 41 must be made of extremely hard material such as tungsten carbide. Pezzoli, U.S. Pat. No. 4,624,288, discloses a mechanism to control the movements of weft insertion members in shuttleless weaving looms as shown in FIG. 5. In such a mechanism, a rotary rectilinear motion of the gearwheel is achieved by a variable pitch cylindrical cam 44 with two pairs of sliding blocks 43 with an involute profile. The first drawback of such a mechanism is that the sliding blocks 43 are difficult to machine. The second drawback of such a mechanism is that the mating precision of the variable pitch cylindrical cam 44 and the sliding blocks 43 is relatively high. The third drawback of such a mechanism is the contact surface of the variable pitch cylindrical cam 44 and the sliding blocks 43 are worn out at high operation speeds. Eventually, the positioning accuracy of the weft-carrying gripper or the weft-drawing gripper deteriorates.